The present invention relates to retransmission techniques in mobile communication systems, in particular CDMA systems and more specifically to a hybrid ARQ (automatic retransmission request) method for packet data transmission that combines previously transmitted packets with retransmitted packets. With each combining operation, redundancy is increased and the packet is more likely to be received correctly even in hostile communication environments.
In more detail, the present invention relates to a hybrid ARQ method according to the preamble part of claim 1. This method is commonly referred to in the art as hybrid ARQ type II or III or incremental redundancy.
A common technique for error detection of non-real time services is based on Automatic Repeat request (ARQ) schemes which are combined with Forward Error Correction (FEC), called hybrid ARQ. If an error is detected by Cyclic Redundancy Check (CRC), the receiver requests the transmitter to send additional bits of data.
From different existing schemes the selective-repeat continuous ARQ is most often used in mobile communications. This scheme in connection with FEC will be used for next generation mobile communication systems such as UMTS. A retransmission unit of the RLC (Radio Link Control) layer is referred to as PDU (Protocol Data Unit).
In the art, commonly three different types of ARQ are defined as specified below. Examples of corresponding prior art documents are:
Performance of punctured channel codes with ARQ for multimedia transmission in Rayleigh fading channels; Lou, H. and Cheung, A. S.; 46th. IEEE Vehicle Technology Conference, 1996;
Analysis of a type II hybrid ARQ scheme with code combining, S. Kallel, IEEE Transactions on Communications, Vol.38#8, Aug. 1990; and
Throughput performance of Memory ARQ schemes, S. Kallel, R. Link, S. Bakhtiyari, IEEE Transactions on Vehicular Technology, Vol.48#3, May 1999.
Type I: The erroneous PDUs are discarded and a new copy of that PDU is retransmitted and decoded separately. There is no combining of earlier and later versions of that PDU.
Type II: The erroneous PDU that needs to be retransmitted is not discarded, but is combined with some incremental redundancy bits provided by the transmitter for subsequent decoding. Retransmitted PDUs sometimes have higher coding rates and are combined at the receiver with the stored values. That means that only little redundancy is added in each retransmission.
Type III: Is the same as Type II with the only difference that every retransmitted PDU is now self-decodable. This implies that the PDU is decodable without the need of forming a combination with previous PDUs. This is useful if some PDUs are so heavily damaged that almost no information is reusable.
Schemes of type II and III are obviously more intelligent and show some performance gain, because they have the ability to adjust the coding rate to changing radio environments and to reuse redundancy of previously transmitted PDUs.
To support incremental redundancy the sequence number SN of the transmission unit has to be encoded separately. The stored data with the known SN can then be combined with subsequent retransmissions.
In prior art the SN is encoded in the PDU header or in the time slot header (e.g. EPA-0938207) and transmitted together with the PDU. If the PDU is corrupted it is likely that also the header is destroyed. Therefore the coding has to be done with a lower coding rate to allow the SN to be read even when the data is erroneous. That means that there will be a large coding overhead to ensure reliable transmission of the sequence number. The coding for the SN therefore has to be different from that used for the PDUs resulting into increased complexity. To ensure that the SN is correct a CRC parity check could be applied, but reliable CRC over few bits is not very efficient.
Besides the signalling overhead that is introduced by the prior art methods it is the implementation complexity that has prevented this technique from being used. A large amount of memory is required in the receiver to store the erroneous packets for combining with the retransmissions. Since the SNs are not known before receiving the retransmission it is not possible to start the combining process before the SNs have been decoded.
The object underlying the present invention is to provide a hybrid ARQ method with less signalling overhead and low implementation complexity.
This object is solved by a hybrid ARQ method as set forth in claim 1.
The present invention overcomes the prior art problems since the sequence number is transmitted over a separate control channel. This allows that the complexity of the receiver is decreased since the sequence number may be transmitted beforehand which allows a more efficient decoding and combining of the PDUs which may follow at a later time. Instead of storing the complete frame, decoding the SNs, combining stored packets with now identified retransmitted packets and finally decoding the packets, only combining and decoding needs to be done. Furthermore, delivery of the SNs on a separate channel easies the introduction of this method into existing systems, since the PDU format and the complete mapping function in the medium access control MAC layer can be left unchanged compared to a retransmission scheme not using type II/III combining.
According to preferred embodiments, different channelisation codes, different time slots and different frequencies are used for the control channel for transmitting the sequence numbers and the data channel for transmitting the PDUs. This provides for additional performance gain obtained due to time and frequency diversity and separate physical channels of the PDU and the SN.
Preferably, the data channel for transmitting the PDUs is a channel shared by several users which allows more efficient use of the channel resources.
According to a preferred embodiment, the control channel for transmitting the SNs is a low rate dedicated channel or a shared control channel in order to save channel resources.
According to a further advantageous embodiment, the quality of service QoS of the control channel is independent from the QoS of the data channel for transmitting the PDUs by suitably controlling at least one of the parameters transmission power, coding rate and spreading factor. Consequently, transmission efficiency as well as reliable transmission of the sequence number is attained by separately controlling the QoS for the SN and the PDU.
For higher data rates, it is advantageous to combine multiple sequence numbers in a sequence number data unit SNDU in order to compress the signalling and to increase CRC efficiency. Preferably, the SNDU is multiplexed with other signalling data or user data to save channel resources. According to a further preferred embodiment the SNDU is sent together with an allocation message on the control channel for a shared uplink or downlink channel transmitting with a high data rate.
Depending on the used physical channel and the access technology, the reception of SNs and the PDUs are either not or less correlated with respect to time. Although it is advantageous that the SNs of the SNDU arrive in the order of the received PDUs, the high rate packet transmissions are less time constrained and allow for a time offset between the SN and the corresponding PDU.
According to a further preferred embodiment, the SNDU is mapped to more than one frame of the control channel which allows interleaving.
Further, it is preferred, that correct reception of a SNDU is indicated from the mobile station to the base station or vice versa as part of a transmission protocol.
If the sequence number is additionally included in the header of each PDU, type III ARQ can be realised.
According to a further advantageous embodiment of the invention, the method includes that a network control unit transmits a signal whether the hybrid ARQ method is to be employed or not. Alternatively, the signal can be transmitted from the mobile or base station. As a variant, the base station and/or the mobile station can recognise from the existence of a SNDU whether the hybrid ARQ method is to be employed or not.